I have a fume problem. I smell exhaust fumes in the cabin. They get pretty heavy. I had the exhausts system checked, no leaks. Some times I can smell gas, but not for long. Checked the plugs and they look good. Can it be the injector? If so, can it be cleaned? Looks like a new one runs around $350! Any suggestions? Really could use help. Thanks forgot- 1991 5 speed 4wheel drive.